Starmer's comments come a day after it emerged that the US and UK had made the decision to allow Ukraine to use partly British-made Storm Shadow missiles to strike targets deeper inside Russia.
ADVERTISEMENTBritain's Prime Minister Keir Starmer has said Vladimir Putin started the war in Ukraine and can end it at any time.
Speaking to reporters on a plane en route to Washington for talks with US President Joe Biden, Starmer said Ukraine has a right to self-defence.
"Russia started this conflict. Russia illegally invaded Ukraine. Russia can end this conflict straight away," he said.
He said the UK had provided "training and capability" to help Ukraine push back Russian troops and said he was visiting Biden because "there are obviously further discussions to be had about the nature of that capability".
Those comments come a day after the British newspaper The Guardian reported that the US and UK had made the decision behind closed doors to allow Ukraine to use partly British-made Storm Shadow missiles to strike targets deeper inside Russia.
But on Thursday, Russian President Vladimir Putin warned that if Ukraine is allowed to use long-range Western-provided missiles to strike targets inside Russia, it would mean the direct participation of NATO countries in the conflict.
Western long-range precision weapons can only be used with intelligence data from NATO satellites and flight assignments entered by NATO military personnel, he claimed.
"This is their direct participation, and this, of course, significantly changes the very essence, the very nature of the conflict. This will mean that NATO countries, the United States and European countries are fighting against Russia," Putin said.
"And if this is so, then, bearing in mind the change in the very essence of this conflict, we will make appropriate decisions based on the threats that will be created for us."
Also on Thursday, Ukrainian President Volodymyr Zelenskyy met with leaders of the Baltic States in Kyiv.
According to the president's press service, the parties discussed the strengthening of military and technical cooperation during a meeting with Latvian Prime Minister Evika Siliņa.
"We face hybrid attacks almost every day from Belarus; now drones are flying over our country, so we are here to learn from you as well," Siliņa told Zelenskyy during the meeting.
Lithuania's President Gitanas Nausėda told Zelenskyy that the West needs to "push away" red lines and allow Ukraine to use Western-supplied weapons against military targets on Russian soil.
"The sooner we understand that we have to push away those red lines that we draw too many times in our heads, the sooner Ukraine's victory will come," Nausėda said.
Military assistance was also on the agenda during the meeting with the Estonian President Alar Karis.
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"We appreciate Estonia's decision to allocate 0.25% of GDP to Ukraine's defence needs every year," Zelenskyy wrote on his social media page following the meeting with Karis.
But while Zelenskyy has continually pushed for permission from western allies to use the weapons they provide to strike inside Russia, he said the world must face up to some uncomfortable questions for the war in Ukraine to end.
"It is very easy to condemn a Russian rocket that flies into our school. But it is not so easy to admit that this rocket can have parts from America, Europe, Asia, from any part of the world," he said at the fourth First Ladies and Gentlemen Summit in Kyiv.